If you face anything like this again, bear in mind that any legal notices you receive which mention potential penalties usually inform the recipient of the
maximum penalties available to a court. Financial penalties are calculated according to the defendant's means and are based on multiples of his net weekly income. Very rarely are maximum financial penalties imposed (and in fact cannot be if the defendant pleads guilty)
Particularly as far as road traffic offences go, the police have "out-of-court" remedies (courses and fixed penalties) available to them for the most common offences which they will invariably offer for all but the most serious of transgressions. Only a minority of speeding allegations end up in court.
To help with your research, here is the guideline document which police use when deciding how to deal with speeding offences:
https://library.college.police.uk/docs/appref/ACPO-Speed-Enforcement-Guidance.pdfIf you scroll to section 9.6 you will see a table showing the thresholds where courses and fixed penalties will normally be offered for each speed limit.